Estimating the length of cable on a reel is more complicated than simply calculating the diameter and width of the reel. You also need to know how many times the cable will wrap around the reel in one layer of cable as well as the number of layers of cable the reel will hold. As the cable winds around the reel, each new layer adds to the diameter, thus each layer consists of more cable than the previous layer. The proper formula will account for this effect by taking the diameter of the cable itself into consideration and increasing the circumference used in the formula with each new layer.

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Measure the diameter of the barrel, or center, of the reel. Multiply this number by pi, or 3.14, to get the circumference of the barrel. For example, the barrel might be 2 inches in diameter, which means the circumference is 6.28 inches.

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Measure the diameter of the cable itself. If the cable is too small to measure with a tape measure, such as speaker wire, use calipers designed to measure small objects. For example, the cable might be one-fourth inch in diameter, or .25 inches.

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Measure the width of the reel. For example, 5 inches.

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Divide the reel width by the cable diameter. For example, 5 divided by .25 equals 20, which is the number of wraps the cable will make around the barrel in a single layer.

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Multiply the reel circumference by the number of wraps to get the length of the first layer. For example, 6.28 inches (circumference) by 20 (number of wraps), which equals 125.6 inches.

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Multiply the diameter of the cable by 2 (in the current example, .25 times 2 equals .5) and add to the diameter of the previous layer (2 inches plus .5 inches equals 2.5 inches), then repeat the rest of the formula to find the length of the next layer. For example, 2.5 inches times 3.14 equals 7.85 inches (circumference). Multiply 7.85 by 20 (number of wraps) to get the length of the second layer--157 inches.

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Measure the flange--the wide, flat piece on each side of the barrel that holds the cable in place--of the reel from the edge of the barrel to the edge of the flange. Divide this number by the cable diameter to find the number of layers the reel will hold. For example, if the flange is 1 inch wide from the edge of the barrel to the edge of the flange, the reel will hold four layers of one-quarter inch cable.

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Perform the calculation above four times to find the total length of the cable on the reel. For this example, the reel holds about 690.8 inches of cable (sum of the lengths of the four layers of cable; the first layer is 125.6 inches, the second is 157 inches, the third is 188.4 inches and the fourth is 219.80 inches).